The invention relates to a process for the production of a component consisting of a fiber reinforced material, with which liquid resin is supplied to a semifinished fiber article by way of application by vacuum pressure.
Such processes are known from the state of the art and are also designated as vacuum injection processes.
Vacuum injection processes are known, for example, from U.S. Pat. No. 4,902,215, U.S. Pat. No. 5,052,906, U.S. Pat. No. 5,601,852, U.S. Pat. No. 5,439,635 or WO 94/20278.
DE 100 13 409 C1 discloses a process for the production of fiber reinforced plastic components from dry semifinished fiber composites by means of an injection process for the injection of matrix material, with which a first chamber is formed by means of a membrane permeable to gas and impermeable to matrix material and a second chamber is formed which abuts on the first chamber which is separated from the surroundings by means of a film impermeable to gas and matrix material and wherein air is drawn off from the second chamber and, as a result, matrix material from a storage vessel is drawn into the evacuated first chamber.
The object underlying the invention is to provide a process of the type specified at the outset, by means of which components of a high quality can be produced and, in particular, components with large surface areas can also be produced.
This object is accomplished in accordance with the invention, with the process specified at the outset, in that a heat curing resin is used as resin and that application by vacuum pressure and temperature are controlled such that in relation to the liquid resin the boiling point curve of the resin is not exceeded.
In accordance with the invention, heat curing resins can also be used for the production of structural components by means of a vacuum injection process. For example, the heat curing resin system Hexcel RTM6 which is authorized for aviation can be used. As a result of the use of low viscosity resins a high fiber volume content in the component can be achieved and this is, for example, between 40% and 60%. The bulk of the component can be reduced due to the high fiber volume content.
On the other hand, heat curing resins normally have a relatively low boiling pressure and so the resin can boil during the heating up due to application by vacuum pressure. The gas bubbles thereby resulting can no longer be removed from the workpiece and thus have a negative effect on the quality of the workpiece. Due to the inventive control of the application by vacuum pressure and the temperature it is possible to avoid the boiling point curve being exceeded. In accordance with the invention, structural components which are practically free from bubbles can therefore be produced with a high degree of repetition.
A separation between the structure of the semifinished fiber article (laminate structure) and impregnation of the resin can be achieved, in particular. In accordance with the invention, structural components with large surface areas can also be produced by means of a vacuum injection process without an autoclave needing to be provided. The costs of manufacturing a component may thus be reduced considerably. For example, foam cores and inserts can also be integrated in one operating procedure.
The impregnation of the semifinished fiber article itself represents a complete process which takes place in a vacuum atmosphere, i.e. is separated from the outside environment. As a result, the workplace stress is low and, in particular, it is possible for no skin contact with resin to occur.
The production of the component may also be carried out in relation to known processes with less time expended. As a result, the costs are again reduced.
It is advantageous when the application by vacuum pressure is controlled via a vacuum pump during the resin infiltration. A pumping capacity of the vacuum pump, which then has a direct effect on the vacuum pressure applied to the workpiece, may be adjusted in a selective manner. Possible losses in pressure in connecting lines and the like can be determined in a simple manner or are known and so during the resin infiltration, during which the temperature of the injected resin and the temperature of the semifinished fiber article are adjusted in a defined manner, a specific vacuum can also be set in order to avoid the resin boiling.
In order to be able to supply the resin to the semifinished fiber article in a defined manner it is particularly favorable when a distribution fabric is provided which has vacuum pressure applied to it. Flow channels are formed in the distribution fabric which serves as a flow aid and a flow of resin is generated in these channels via a pressure gradient or capillary effect. As a result, the workpiece may be impregnated with resin in a selective manner, wherein the resin flow and, in particular, resin flow fronts can be controlled.
It is favorable when the pressure following the resin infiltration is measured at a distribution fabric which serves to supply resin to the semifinished fiber article. Since the distribution fabric normally has a high permeability and covers the entire workpiece, a take-up of pressure at the distribution fabric is particularly suitable for carrying out a pressure control.
In order to measure the pressure it is advantageous when one or several pressure sensors are brought into operative connection with the distribution fabric following the resin infiltration of the workpiece. The prevailing pressure may then be determined via these pressure sensors. In order to prevent resin passing into connecting lines to the pressure sensor or sensors during the infiltration of the workpiece with resin (impregnation), the operative connection is interrupted prior to and during the resin infiltration of the workpiece. In this phase, the pressure is, in particular, adjusted via the capacity of the vacuum pump in order to avoid any boiling of the resin accordingly. Following the resin infiltration, the operative connection is again established in that a line clamp is, for example, released. The pressure at the workpiece then prevails at the pressure sensor and the vacuum can be adjusted accordingly at a given temperature with the aid of the measurement value of the pressure sensor such that no gas bubbles are formed in the resin.
In order to produce certain structural components it may be provided for the semifinished fiber article to be placed in a mold during the resin infiltration. In this respect, it is sufficient to design the mold as a one-sided form, onto which the semifinished fiber article is placed. A vacuum chamber, in which a vacuum prevails, may then be established by means of a vacuum foil which is laid over the semifinished fiber article. No specially designed autoclave has to be provided for manufacturing the structural component and, in particular, components with large surface areas can also be produced.
In order to ensure a high quality of the component it is provided, in particular, for the temperature of the mold to be controlled. As a result, the temperature of the resin during the infiltration (impregnation) can, on the one hand, be controlled in order to avoid the occurrence of boiling bubbles. On the other hand, an optimum temperature may be set for the curing of the resin after the infiltration in order to again ensure a high quality of the component in this case, as well.
To monitor the temperature it may also be provided for a plurality of temperature sensors to be arranged at a vacuum foil. The vacuum foil is arranged above the workpiece so that a measurement of temperature is also made possible in this area.
It is particularly advantageous when the temperature is adjusted with respect to the temperature dependence of the viscosity of the resin. If the viscosity is too high there is the risk of resin, which saturates the semifinished fiber article, flowing out again. If, on the other hand, the viscosity is too low, the semifinished fiber article will not be sufficiently impregnated.
A resin infiltration favorably takes place in an injection phase at a certain temperature or in a certain temperature range, at which the resin has such a viscosity that an essentially uniform resin front can be formed. The resin front as flow front then passes uniformly through the semifinished fiber article and this is impregnated uniformly without any air and gas pockets being formed. As a result, a high quality of the component can again be achieved. In practice, it has proven to be favorable when the temperature is adjusted such that the viscosity of the resin is in the range between 10 mPas and 1000 mPas and, in particular, between 20 mPas and 500 mPas.
During the curing of injected resin, the temperature can be increased to aid the curing. On the other hand, exothermal resin systems such as Hexcel RTM6 are also known which cure exothermally. In order to avoid any boiling of the curing resin, it is therefore advantageous when a reduction in the application by vacuum pressure takes place in a curing phase which follows an injection phase. As a result, resin is also prevented from being withdrawn from the workpiece. Furthermore, the risk of the resin outgassing is also reduced as a result.
It is favorable for the defined curing and gelling of the resin when an increase in temperature takes place in the curing phase which follows the injection phase.
It is particularly advantageous when the temperature is increased in a curing phase, in which the resin is completely cured, in order to thus achieve the complete curing of the resin. The temperature is thereby increased in relation to an injection phase. It is favorable for the injection phase when the temperature is set to be relatively low in order to be able to set a high underpressure (good vacuum). In the curing phase, the semifinished fiber article is already saturated with resin and as the pressure can then be increased (vacuum can be reduced), an increase in temperature can also be carried out for the curing.
Furthermore, it is particularly advantageous when the temperature in the final curing phase is also increased in relation to a curing phase which follows an injection phase. In the final curing phase the resin is already cured to such an extent that boiling bubbles are no longer formed and so an increase in temperature is possible even without an additional reduction in pressure. The resin is reacted out completely due to the increase in temperature.
It is favorable from the point of view of manufacturing techniques when the temperature is adjusted such that a certain processing time or a certain processing period of time is specified for the resin. Apart from adjusting and monitoring pressure and temperature and, as a result, the viscosity, the processing period of time for the resin must also be taken into consideration as this gels, wherein the gelling time is, again, dependent on the temperature. If the temperature is selected to be too high, the resin will, in certain circumstances, gel too quickly without a uniform impregnation of the semifinished fiber article having, for example, been achieved. The processing period of time is adjustingly adapted to the size of the component.
In accordance with the invention, it is provided, in particular, for a process monitoring with respect to resin infiltration and resin curing to be carried out. As a result, a high quality of the component can be achieved in that, on the one hand, the occurrence of boiling bubbles during the resin infiltration and resin curing is avoided and, on the other hand, the drawing of resin out of the impregnated semifinished fiber article during the curing phase is avoided. The outgassing of resin during the curing phase may also be avoided. In addition, a defined resin curing can be carried out.
In a variation of one embodiment it is provided for the resin to be pre-aged prior to the infiltration in order to increase the viscosity. For this purpose, the resin is kept for a certain time at a certain temperature or in a certain range of temperatures. When this pre-aged (prereacted) resin is then supplied to the semifinished fiber article, it is less highly fluid and the problem of the resin xe2x80x9cflowing throughxe2x80x9d the semifinished fiber article during the impregnation or the problem of the resin being drawn out of the impregnated semifinished fiber article is reduced.
A resin trap is favorably provided, by means of which a uniform application by vacuum pressure is made possible after the resin infiltration and which essentially prevents any removal of resin by suction during a curing phase of the resin. As a result of such a resin trap, the quality of the component can be ensured after the impregnation since no resin is drawn off and, on the other hand, the application by vacuum pressure still remains uniform.
The resin trap favorably comprises an extraction guide means which has such a large internal diameter that air and gas bubbles can rise without resin being pressed into an extraction chamber. As a result, vacuum pressure can be applied to the distribution fabric; on the one hand, the uniformity of the application of vacuum pressure to the workpiece is promoted and, on the other hand, resin is removed at the most from the distribution fabric but not from the workpiece. If the extraction guide means itself has vacuum pressure applied to it, the uniformity of the application of vacuum pressure to the workpiece can be aided as a result. For example, a storage vessel for resin may be used as extraction vessel following the infiltration in that a cover is closed in an air-tight manner. The vacuum pressure (underpressure) at the workpiece within a vacuum chamber is then likewise present in the extraction chamber in order to promote the uniformity of the application by vacuum pressure over the workpiece. On the other hand, the extraction guide means prevents the resin from being able to ascend into the extraction vessel. It is, therefore, favorable when the extraction chamber is formed in a storage vessel for resin for the resin injection.
In an advantageous variation of one embodiment, a distribution fabric which serves as a flow aid for the supply of resin to the semifinished fiber article is no longer operative after a certain-distance in relation to a workpiece edge. When resin passes into the semifinished fiber article from the distribution fabric and, in particular, passes in at right angles to laminate layers of the semifinished fiber article, a flow front is formed with a given angular course of the flow front. If the distribution fabric is still operative at the workpiece edge, oppositely directed flow fronts can form which can lead to air pockets which can influence the quality of the component. As a result of the fact that the semifinished fiber article is no longer operative after a certain distance in relation to a workpiece edge, the flow front angle can be increased and, in particular, it can, in the ideal case, be set vertically to layer surfaces of the laminate structure. As a result, no air pockets can occur.
In practice, it has proven to be favorable when the distance is in the range of between 10 mm and 50 mm and, in particular, between 25 mm to 35 mm. The distance set each time depends on the thickness of the component.
In a variation of one embodiment, the flow aid is made inoperative in the area at the workpiece edge in that the distribution fabric ends before the given distance so that the distribution fabric does not, therefore, end flush with the workpiece edge. The speed of the resin is then reduced considerably in the area without any distribution fabric which leads to an increase in the flow angle of the resin front.
In an alternative embodiment, a cover film is provided between workpiece and distribution film to limit the effectiveness of the distribution fabric, whereby the supply of resin to the semifinished article is again hindered and as a result the speed of the resin is again reduced; in this way, a steeper resin front can also be formed.
The problem of air pockets can also result at workpiece edges, at which laminate layers meet one another at an angle. The course of the resin front is bent at such an edge which can, again, lead to air pockets. In accordance with the invention, it is provided for the distribution fabric which serves as a flow aid during the supply of resin to be cut in relation to a workpiece edge in order to control the angular course of the flow front of the resin. By cutting the distribution fabric at the edge, a gap results between pieces of distribution fabric which are associated with the laminate layers which meet one another at an angle. The speed of the resin flow is then reduced again at the edge itself which leads to a steeper angle of the resin flow front, whereby the risk of air pockets is again avoided to a great extent.
In order to see to it that the resin does not flow around an edge with a steeper angular course of the flow front such that oppositely directed flow fronts can, for example, be formed, it is particularly advantageous when an edge end is sealed in order to prevent any flow around it.
It is particularly favorable when one or several vacuum ports are provided, via which the workpiece has vacuum pressure applied to it and which is or are connected to one or several vacuum pumps. The vacuum in a vacuum chamber which is limited, in particular, by a vacuum foil may be introduced via the vacuum port.
In this respect, one vacuum port is favorably arranged in an area which is reached by a flow front of the injected resin last. A complete impregnation can be ensured as a result.
The inventive process may be used inexpensively when one vacuum port is designed as a resin trap which can accommodate a certain amount of resin in order to prevent resin passing into a vacuum system. It may happen that resin will been drawn in by the vacuum port. Due to its design as a resin trap the vacuum system will not be soiled.
In a variation of one embodiment it is provided for the vacuum port to be connected via distribution fabric to an underside of the semifinished fiber article. The vacuum pressure is then applied to the semifinished fiber article, wherein it is possible, on the other hand, by means of corresponding seals to avoid resin being drawn in via the vacuum port from an upper side of the distribution fabric.
It is favorable when a connection of the vacuum port to a vacuum foil is sealed in order to be able to form a vacuum chamber.
Furthermore, it is favorable when the distribution fabric is sealed in relation to a workpiece edge so that resin cannot be drawn off upwards by means of the vacuum port. Furthermore, it is also favorable when a film is arranged between distribution fabric and seal and when the vacuum port is sealed in relation to the workpiece. The workpiece then has vacuum pressure applied to it essentially only via the distribution fabric which is connected to an underside of the semifinished fiber article. The possible penetration channels, via which resin can reach the vacuum port, are then minimized.
It is particularly favorable when a resin brake is arranged at a workpiece edge. In the case of low-viscous resin (highly fluid resin) there is the risk that resin will be injected into a semifinished fiber article but that this can partially flow out again or can be drawn off. This can be reduced or rather avoided by means of the resin brake as stop area for the flow of resin.
It is favorable when a first connection for the application by vacuum pressure is arranged in front of a resin brake in relation to the semifinished fiber article and a second connection is arranged behind a resin brake. The uniformity of the application by vacuum pressure may then be achieved during the resin infiltration via the first connection. The vacuum is maintained via the second connection even after the impregnation of the semifinished fiber article while the first connection is uncoupled (the application by vacuum pressure is stopped) when the resin front reaches it.
In a variation of one embodiment it is provided for a process monitoring to be carried out by means of ultrasound acting on the workpiece. This may be an on-line process monitoring, wherein the sound velocity and the attenuation in the workpiece, in particular, are determined. As a result, the degree of curing of the resin can, for example, be determined, and the quality of the workpiece can also be monitored.
A polyaddition resin, such as an epoxy resin or a bismaline resin, is used, for example, as heat curing resin.
Furthermore, it is favorable when the resin supply speed of resin from a resin store to the semifinished fiber article can be controlled in order to obtain a good impregnation of the semifinished fiber article with resin.
The object cited at the outset is solved, in addition, by an apparatus for carrying out the process which comprises at least one vacuum port for applying vacuum pressure to a workpiece, wherein the vacuum port is designed as a resin trap which can accommodate a certain amount of resin in order to prevent the resin from passing into a vacuum system which is connected to the vacuum port.
This apparatus has the advantages already explained in conjunction with the inventive process.
Additional, advantageous embodiments are the subject matter of the subclaims which follow the corresponding apparatus connection. Advantages of these embodiments have likewise already been explained in conjunction with the inventive process.
The object cited at the outset is accomplished, in addition, by an assembly for the production of a component consisting of a fiber reinforced material by means of resin impregnation of a semifinished fiber article which comprises:
a mold;
a vacuum foil, by means of which a vacuum chamber can be produced, in which the semifinished fiber article can be positioned on the mold, wherein vacuum pressure can be applied to the vacuum chamber and
a device for supplying liquid resin to the semifinished fiber article,
wherein, in accordance with the invention, application by vacuum pressure and temperature can be controlled during the resin impregnation such that in relation to the liquid resin the boiling point curve is not exceeded.
This assembly has the advantages already explained in conjunction with the inventive process and the inventive apparatus.
Additional, advantageous embodiments have already been explained in conjunction with the inventive process and the inventive apparatus.